Multi-responsive paper chemosensors based on mesoporous silica nanospheres for quantitative sensing of heavy metals in water

Exposure to low concentrations of heavy metal cations seriously harms living organisms, hence they are considered environmental toxins. Portable simple detection systems are required for field monitoring of multiple metal ions. In this report, paper-based chemosensors (PBCs) were prepared by adsorbing 1-(pyridin-2-yl diazenyl) naphthalen-2-ol (chromophore), which recognizes heavy metals, onto filter papers coated with mesoporous silica nano spheres (MSNs). The high density of the chromophore probe on the surface of PBCs resulted in ultra-sensitive optical detection of heavy metal ions and short response time. The concentration of metal ions was determined using digital image-based colorimetric analysis (DICA) and compared to spectrophotometry under optimal sensing conditions. The PBCs exhibited stability and short recovery times. The detection limits determined using DICA of Cd2+, Co2+, Ni2+ and Fe3+ were 0.22, 0.28, 0.44, and 0.54 μM; respectively. Additionally, the linear ranges for monitoring Cd2+, Co2+, Ni2+ and Fe3+ were 0.44–4.4, 0.16–4.2, 0.8–8.5, and 0.002–5.2 μM; respectively. The developed chemosensors showed high stability, selectivity, and sensitivity for sensing of Cd2+, Co2+, Ni2+ and Fe3+ in water under optimum conditions and hold potential for low cost, onsite sensing of toxic metals in water.


Introduction
Heavy metal ions leak into surface or underground water from industrial activities or geological sources and pose a severe risk to living organisms. 1 Cadmium is widely used in different industries including metal alloys, electroplating, stains, fertilizers, and rechargeable batteries. 8 Accumulation of cadmium in tissues has been correlated to the enlargement of vital organs, bone weakening, and impairment of the cardiovascular, immunological, and reproductive systems. 9 Cobalt is used as a catalyst for chemical industries and manufacturing rechargeable lithium-ion batteries. 10 Although cobalt plays an important function in mammalian metabolism as a trace element; exposure to high cobalt levels has been linked to cardiomyopathy and thyroid hyperplasia. [11][12][13][14] Iron is an essential component of key proteins involved in major physiological functions such as respiration and energy metabolism. 2 Iron overload, however, leads to damage of vital organs such as liver, heart, and endocrine glands. Nickel is widely used in making Ni-Cd batteries and electroplating. 3 Exposure to high levels of nickel has been linked to cardiovascular, kidney, and lung diseases. [4][5][6][7] Multiple guidelines for acceptable levels of heavy metals in drinking water were published by different regulatory bodies including the US Environmental Protection Agency, the European Union, and the World Health Organization (Table  1). [15][16][17][18][19] Various analytical techniques have been established for detection of heavy metal ions such as ICP-AES, ICP-MS, chemiluminescence, atomic absorption spectrometry, 20,21 solidphase 22,23 and uorescence spectroscopy. 24 However, they have several drawbacks such as long turnaround time, complicated procedures, need for sophisticated infrastructure, and high cost. Therefore, the challenges for sensing different heavy metals using chromogenic materials and portable chemosensors were investigated in different assays. [25][26][27][28][29][30][31][32][33] In this report, multi-responsive PBCs were prepared for monitoring of Cd 2+ , Co 2+ , Fe 3+ , and Ni 2+ in water. A chromophore probe was immobilized onto mesoporous silica nanosphers (MSNs) for selective detection of various heavy metal ions. 34,35 MSNs offer several advantages, including large pore volume, high surface area, and homogeneous mesopore size distribution. To generate quantitative results, color generated by the chemosensors upon detection of heavy metals was quantied by digital image colorimetric analysis (DICA) and compared to UV-

Morphology of PBCs
Field emission scanning electron microscopy (FESEM) was used to examine the morphology of the lter paper and the lter paper coated with MSNs ( Fig. 1A and B). A thin layer of MSN was observed which covered the pores of the cellulose paper. Cellulose bre cages were blocked aer decoration with chromophore ( Fig. 1C and D). SEM-EDS analysis and mapping of treated lter paper with MSNs, paper based chemosensors (PBCs), and PBCs + M 2+ were investigated. The existing of Si element was conrmed the treatment of lter paper with MSNs ( Fig. S1A †). The distribution of chromophore elements and metal ions over treated lter paper with MSNs conrms the successful immobilization and complexation respectively ( Fig. S1B and C †). The X-ray diffraction patterns of paper with MSNs and PBCs were analyzed (Fig. 1E). The lter paper containing MSNs, PBCs, and PBCs aer detection of metal ions revealed specic diffraction peaks ascribed to (110), (200), (110) and (004) planes. The cellulose structure was identied by the resulting XRD data. 37 The diffraction intensities of the paper containing MSNs reduced signicantly aer chromophore decoration. Results suggest hydrogen bond interactions between the cellulose chains with MSNs and the chromophore. 38 FTIR spectroscopy was conducted to investigate the decoration of chromophore into lter paper (Fig. S2 †). The existence of band at 797.97, 1053.78, which are referred to siloxane bond (797.97, Si-O-Si bending (1053.78 cm −1 ). Our  result conrms the successful graing of MSNs. The stretching of the -OH group is responsible for the wide band between 3400 and 3200 cm −1 , whereas the C-H stretching of methylene groups is responsible for the band about 2903 cm −1 . The N 2 isotherms and NLDFT experiments were conducted to determine the pore size and surface area of the fabricated PBCs and their complexes (Fig. 1). [39][40][41] Untreated lter paper, paper containing MSNs, optical probe, and complex showed the same isotherm type (III). The surface area of paper containing MSN was 5.79 m 2 g −1 and that of untreated paper was 1.95 m 2 g −1 , indicating that MSNs were successfully adsorbed to the lter paper. The PBC surface area decreased dramatically (1.31 m 2 g −1 ) when they were in combination with metal ions (0.91 m 2 g −1 ), which suggests the efficient loading of chromophore on the treated lter paper. The NLDFT investigations revealed the presence of different pore sizes in the lter paper containing MSN (Fig. 1G). Aer the immobilization procedure, the pore diameter of the treated lter paper was dramatically decreased.

Determination of optimum pH for sensing metal ions
Changes in pH dramatically modify color intensity and dispersion at ultra-trace metal ion concentrations. Standard solutions of metal ions were prepared using buffer solutions at different pH values and the UV-vis absorbance was measured to identify the optimal pH level for metal ion sensing using PBCs. The determined optimal pH values for sensing Fe 3+ , Co 2+ , Ni 2+ and Cd 2+ were 5.5, 7.0, 9.0 and 10; respectively (Fig. 2).
Quantitative measurement of metal ions using PBCs and spectrophotometry UV-vis absorption spectra were obtained for PBCs treated using increasing concentrations of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ . Then the detection ranges (D R ) of each metal ion sensing technique were determined (Fig. 3). PBCs kits offered one-step detecting methods for both qualitative and quantitative determination of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ .
The calibration curves of PBCs were linear at low concentration ranges of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ (Fig. 4). The determined limits of detection (L D ) suggest that the fabricated PBCs have recognized ultra low concentrations of target ions, as compared to sensors fabricated by conventional strategyies. PBCs based on MSNs enabled, for the rst time, efficient metal ion detection down to ∼10 −9 mol L −1 ( Table 2). The fabricated PBCs exhibited greater recognition of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ compared to other chemosensors that have been described previously (Table 3).
Quantitative determination of metal ions using PBCs and digital image-based colorimetric analysis (DICA) The change in RGB intensity values (IR, IG, and IB) of PBCs were examined using the images of PBC colors (obtained using cell     (Table 4). As shown in Fig. 5, the relationships between color absorbance and the concentrations of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ agreed with those obtained by spectrophotometry (Fig. 3). The Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ concentrations and the absorbance of the mean integer value for each RGB component were shown to be linearly correlated, demonstrating that these ions may be detected with great sensitivity at extremely low concentrations (Fig. 6). The L D and LQ of metal ions using DICA are in agreement with those obtained by spectrophotometry.
Therefore, DICA can be utilized as a low cost, portable, and semi-quantitative analytical method for sensing Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ recognized by the developed PBCs.

Selectivity analyses
The selectivity of PBCs towards Cd 2+ , Co 2+ , Ni 2+ , and Fe 3+ was investigated under optimal sensing conditions. Known concentrations of possible interferring cations were added to 0.5 ppm of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ and solutions tested using the PBCs at the specic ion sensing pH values of 5.5, 7, 9, and 10 for Fe 3+ , Co 2+ , Ni 2 and Cd 2+ ; respectively. For each target ion, and in the presence of interfering ions, there were no signicant variations in the PBC absorption spectra or visual color patterns as detection of metal ions was carried out at their specic pH values (Fig. 7). Table S1 † illustrates the tolerance concentrations of several interfering ions.

Proposed sensing mechanism of PBCs
Under optimum sensing conditions, commercial lter papers treated with MSNs then covered with the optical probe were employed to detect Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ (Scheme 2). When oxygen and azo-nitrogen of organic probe on PBCs are available for complexation with Co 2+ , Cd 2+ , Ni 2+ , or Fe 3+ , stable complex with two coordination spheres are formed. As indicated in Scheme 2, the stoichiometric ratio of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ to organic probe at a specic pH is predicted to be 1 : 2. The results show that raising Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+  concentrations improved absorption spectra (Fig. 3). Furthermore, complex formation and charge transfer were linked to the occurrence of an isosbestic point. The developed PBCs enable naked-eye detection of ultra-trace metal ion concentrations without the need for complicated traditional techniques.

Using PBCs for detection of metal ions in water
PBCs were used to test several drinking water samples to identify Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ . Water samples were collected from a variety of places and spiked with known concentrations of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3 . Under optimum sensing conditions, PBCs were dipped into the water samples and different colors developed immediately in presence of different metal ions (Table S2 †). Water samples were tested three times for Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ , then the results were quantied using DICA. Results obtained from DICA analysis were concordant to those obtained by spectrophotometry (Table S2 †). Elution studies were carried out using various concentrations of EDTA to determine the best eluent for adsorbent renewal and reusability (Fig. S3 †). The designed optical chemosensors for sensing Fe(III), Co(II), Ni(II), and Cd(II) ions can be used multiple times.
To the best of our knowledge, this study is the rst to develop multi-responsive optical chemosensors based on mesoporous silica that incorporates simplicity but still retains sensitivity, selectivity, and fast-response detection of target ions in drinking and environmental water samples. Coating the lter papers with mesoporous silica nanospheres as carriers of the probe improved the sensing functionality for detection of ultratrace concentrations of metal ions in water samples. The optical chemosensors have uniform structural morphology enabled by utilizing mesoporous silica nanospheres as carriers for sensing Table 4 Recognition of metal ions using PBCs and digital image-based colorimetric analysis   Effect of common interfering cations on absorbance spectra of paper-based chemosensors under optimum sensing conditions (Co 2+ at l 570nm , Ni 2+ at l 568nm , Cd 2+ at l 570nm , and Fe 3+ at l 558nm) .
the ultra-trace concentration of metal ions in water resources. The detection limit of sensing ultra-trace metal ions is 100 times lower than other reported method. The developed optical chemosensors can be regenerated, using 0.1 M EDTA solution, and reused multiple times. Superior selectivity of optical chemosensors for their target cations under optimal sensing conditions (pH, etc) even in the presence of multiple interfering cations. Colorimetric techniques can be used to measure color intensities of the chemosensors and generate quantitative results. The fabricated optical chemosensors exhibited high stability (in water and on shelf). Clearly, this study supports several of the United Nations Sustainability Development Goals including good health and well-being, clean water and sanitation, and industry and innovation. The designed paper based chemosensors (kits) are of low cost compared to similar commercial ones which would enable large scale accessibility by populations in low-and middle-income countries. It is also of note that the produced chemosensors can be regenerated and used multiple times. Additionally, similar chemosensors can be optimized for detection of toxic metals in cosmetics. Finally, future studies can investigate the use of the developed strategy for removal of heavy metals from water.

Fabrication of PBCs
MSNs were prepared at pH = 7 and applied on lter papers as described previously with minor modications. 28 CTAB (0.5 g) was mixed with 100 mL of distilled water for 30 min. Ethanol (10 mL) and TEOS (2.5 mL) were then added and the solution mixed for another 0.5 h (Scheme 1A). Then, 1.5 mL of NaOH was added and the solution stirred for 2 h. The lter papers were immersed in the prepared MSN solution multiple times (5 min each). The papers containing MSN were dried at 50°C for 6 h then dipped in ethanolic solution (30 mL) of 1-(pyridin-2-yl diazenyl) naphthalen-2-ol (100 mg) for multiple times (5 min each) as shown in (Scheme 1B). To produce the PBCs which were then cut into appropriate size (1 cm 2 ) and placed into 3D printed holders (Scheme 1C).

Analysis of toxic metal ions
Metal ion solutions (200 ppm) were prepared in Milli-Q water. The PBC was placed in a quartz cuvette then a known concentration of a specic metal ion was added and sonicated for 5 s. UV-vis spectroscopic spectra were measured within seconds (without shaking). Colorimetric detection of different concentrations of Cd 2+ , Co 2+ , Fe 3+ , and Ni 2+ using PBCs were performed at different pH. UV-vis spectra were obtained and aer equilibration, a prominent color change was observed and the PBC absorbance spectra obtained. The color change of PBCs was also quantied using DICA.

Conclusions
PBCs were fabricated by adsorbing mesoporous silica nanosphere carrier, prepared using a low-cost method, on lter papers then decorated with the optical chromophore 1-(pyridin-2-yl diazenyl) naphthalen-2-ol. Optimal sensing conditions for each PBC were determined. The MSN-based PBCs enabled visual detection of Co 2+ , Cd 2+ , Ni 2+ , and Fe 3+ present in water with high sensitivity and selectivity. The PBCs allowed detection of multiple ions and featured long-term stability and could be utilized several times using a simple regeneration process. Quantitative results were obtained by analysis of images (obtained using cell phone camera) of colored PBCs using DICA which were similar to those obtained with spectrophotometry. This possibly is the rst report that employs PBCs based on MSNs for detecting multiple heavy metal ions in water using DICA or UV-vis spectroscopy. Additional PBCs could be developed using similar strategies and optimized for detection of additional metals in water and other matrices.

Author contributions
Conceptualization, I. M. E, H. M. E. A.; methodology and data collection, A. R. and I. M. E.; writing, reviewing and editing, all Scheme 2 . Schematic diagram of the possible interactions of the paper-based chemosensors (PBCs) with Cd 2+ , Co 2+ , Fe 3+ and Ni 2+ under optimal sensing conditions. Upon the formation of different complexes between the optical probe and the heteroatom, the color of PBCs changes: (a) from yellow to pink in presence of Cd 2+ at pH = 10; from yellow to dark green in presence of Co 2+ at pH = 7, (c) from yellow to red in presence of Ni 2+ at pH = 9, and (d) from yellow to dark brown in presence of Fe 3+ at pH = 5.5.